What do adoption/twins studies show?
Here is an accelerated version of a conversation I had with Bryan Caplan, noting that I do not pretend to be representing his views but rather I am writing in broad brush terms.
"Culture" and "genes" are two major factors determining individual outcomes, toss in parenting, and if you wish call parenting and culture two parts of "environment." It is obvious that culture matters a great deal, and this comes from knowledge which existed prior to rigorous behavioral genetic studies.
I say "soda" and people in Nebraska say "pop." Singapore vs. southern China. German musical tastes in 1780 vs. today. Rural Africa vs. urban Africa. Most concretely, if I meet someone I want to know what country he came from and grew up in; in fact that is the first thing I wish to know. "The culture word" may be overused and abused, but still the power of culture is evident.
If twin adoption studies seem to show that parenting does not matter much, I think:
1. Matter for what and for whom? Parenting matters a lot for language and religion and obedience and also one's sense of "how the world works," and those factors matter to parents even if they don't always matter to researchers and economists. The word "matters" is going to carry real weight here; in my admittedly extreme pluralist view, "doesn't affect adult income" does not translate into "does not matter." Ask yourself how easily you could marry someone from a very foreign culture -- it matters.
2. We already know that culture matters a great deal in shaping what kind of adults children become, but often individual families cannot much affect the broader culture a child is raised in. It's sometimes the individual family which is impotent, not the surrounding culture as a whole.
3. Most parents are deep conformists. There isn't always a lot of cross-sectional variation in adoption studies. Even if most parenting strategies don't matter (if only because they are not varying much), if a child is raised in an Orthodox Jewish family, or in a strict American-Chinese family, or among the Amish, that probably matters, even adjusting for genes.
The adoption studies can be showing that a) most parents don't so much shape a child's culture at the margin, or b) that environment doesn't much matter in light of the power of genes.
I see a lot of leaping at b), when to me a broader look at the social sciences -- and also some personal travel -- shows a) to be important.
While I tend to see genetic influences as very powerful, the adoption studies don't much push me any extra mile in this regard. In general, I see some proponents of the adoption studies as putting them forward as the most convincing form of evidence, and mentally working to downgrade the relevance of other forms of evidence, including the vast body of knowledge in sociology and social anthropology. I would prefer a more pluralistic approach which draws in many bodies of knowledge at the beginning, admittedly with a messier overall understanding, but that is a good rather than a bad thing.
Now to speak on behalf of Bryan for a moment, and in a way he might object to, I wonder if he isn't leaping at b) too quickly, rather than weighing a) vs. b), using evidence which does and indeed cannot not come from twin adoption studies. The adoption studies don't actually adjudicate a) vs. b) and they don't rule out great power for the broader environment.
Another way to put the point is to examine Judith Harris's claim that genes and peer selection are what shape children. Is the claim about peers -- which falls out of the statistics -- causal? Or are peers best thought of as a sufficient statistic for the broader surrounding culture, thereby placing the causal force in that culture?
If it is the latter, the Harris evidence is simply showing, once again, that both genes and culture matter. Which is fine, but it's hardly a revelation. It also leaves open the possibility that parents who wish to influence their children simply need to try harder to shape their surrounding culture. The Amish are not the only ones who succeed in that endeavor, even if most people do not succeed or wish to try very hard.
You will find Bryan's views on these issues in his forthcoming book. Other readings on what twin studies mean are here (pdf), here, here (Bryan's twins spend a lot of time together!), and here (pdf, a very good piece by Kamin and Goldberger). My favorite bit is the very last paragraph, which I will put under the fold: